During the current funding period, we made significant progress defining the role of CXC chemokine receptor 3 (CXCR3) in the pathogenesis of septic shock. Our results show that CXCR3 is an important regulator of natural killer cell trafficking during septic shock caused by cecal ligation and puncture (CLP). Furthermore, CXCR3 deficiency or blockade causes significant attenuation of sepsis-induced physiologic dysfunction, systemic cytokine production and survival. Deficiency or blockade of the CXCR3 ligand CXCL10 also conferred protection from CLP-induced septic shock. Survival benefit was achievable when anti-CXCR3 IgG or anti- CXCL10 IgG was administered with fluids and antibiotics at 6 hours after CLP. Therefore, CXCR3 blockade was effective when initiated after the onset of sepsis and may represent a viable addition to a multi-modal approach for the treatment of septic shock. Yet, several questions remain regarding the efficacy of CXCR3 blockade in clinically relevant models of sepsis and the mechanisms by which the CXCR3 axis is activated during sepsis. The goals of this research project are two-fold: 1) To evaluate clinically relevant approaches for CXCR3 blockade with the goal of bringing this approach closer to application in the clinical setting and 2) To define the mechanisms by which CXCR3 activation facilitates the pathogenesis of septic shock and determine the cellular and molecular mechanisms by which the CXCR3 axis is activated during sepsis. Specific Aim 1: To define the efficacy of CXCR3 blockade in clinically relevant models of sepsis. In this aim, we will evaluate the efficacy of CXCR3 blockade when initiated after the onset of severe sepsis as indicated by the development of hypoperfusion and organ injury. The CLP model will be utilized as well as our well-developed models of Pseudomonas aeruginosa and Staphylococcus aureus burn wound sepsis. The efficacy of the small molecular weight CXCR3 inhibitor AMG487 will be determined in addition to immunoglobulin-based strategies. Specific Aim 2: To determine the role of the CXCR3 axis in the host response to infection. Some recent studies indicate that CXCL10 plays a role in facilitating effective bacterial clearance mechanisms in less severe models of systemic infection. Studies proposed in this aim will determine whether CXCR3 axis blockade will impair the host response to local infection and whether blockade of the CXCR3 axis during the acute phase of severe sepsis will predispose the host to secondary infections. Specific Aim 3: To define the mechanisms by which the CXCR3 axis is activated during septic shock and define the cell populations responsible for CXCR3- mediated pathobiology. These studies will define the mechanisms that regulate CXCL10 production in vivo. Emphasis will be placed on evaluating the importance of interferon signaling pathways. Further studies are proposed to determine the functional importance of CXCR3-mediated lymphocyte trafficking during severe sepsis and whether CXCR3 serves as an activating factor for non-migratory lymphocyte populations.